The phosphatidylinositol 3-kinase (PI3K) signaling pathway transduces signals for growth, proliferation and cell survival. The tumor suppressor PTEN (phosphatase and tensin homolog on chromosome 10) is the predominant negative regulator of the PI3K pathway and the second most frequently mutated gene in human cancer. Somatic inactivation of PTEN occurs in diverse human tumors including glioblastoma multiforme (GBM), and inherited mutation of PTEN has consequences in multiple organ systems including growth defects and functional abnormalities in the nervous system. Recently, activating mutations in PIK3CA, encoding the catalytic subunit of PI3K alpha (p1101), were identified in GBM and several other tumor types, providing additional evidence of the central importance of the PI3K signaling pathway in human cancer. The long-term goal of these studies is to determine PTEN and PIK3CA function in growth regulation and tumorigenesis in the brain. We hypothesize that Pik3ca mutation may provide a different level of PI3K pathway activation than Pten loss of function and that the extent of PI3K pathway activation as well as the cellular background and presence of cooperating mutations, will determine the outcome. We will develop a novel mouse model in which mutation of Pik3ca can be induced in vivo by cre recombinase. This model will be used along with models of cre-inducible inactivation of Pten developed in our lab to determine how Pik3ca mutation alone and in combination with Pten inactivation influences proliferation, survival and growth in astrocytes and in an in vivo model of high-grade astrocytoma. We will test the response of transformed astrocytes to inhibitors of the PI3k signaling pathway in vitro and in vivo. We will investigate mutations that cooperate with dysregulated PI3K signaling in astrocytoma. Detailed analysis of how PIK3CA and PTEN regulate key effectors within the PI3K pathway in different contexts will greatly increase our understanding of this pathway and may help to predict how tumors with different mutations would respond to small molecular inhibitors that block the pathway at different levels. RELEVANCE TO PUBLIC HEALTH: The PI3K pathway plays an important role in brain tumors as well as other cancers. Regulation of this pathway is disrupted by mutations in PTEN and PIK3CA. We will perform a detailed analysis of how these mutations disrupt normal function and contribute to brain tumor formation. The proposed experiments will provide improved insights into the regulation of PI3K signaling in brain tumors that will be essential for informed design of therapeutic inhibition of this pathway in tumors with distinct mutations.